A spirit form is a transformation that occurs to a player character after death. Most races become a ghostly translucent version of their normal appearance, but night elves transform into a wisp (despite their other similarities, blood elves do not turn into wisps). As a spirit you can move faster, walk on water and go through locked doors/gates to get into the instances where your corpse is (but not actually go into the instance). In some cases (like in Eastern Plaguelands or Icecrown) you will spawn a spirit mount to travel to your body easier.